The invention relates generally to X-ray imaging and in particular to X-ray imaging using monoenergetic X-ray sources and energy discriminating detectors.
X-rays have found widespread application in various non-invasive medical and non-medical imaging techniques. In general, X-ray based imaging systems direct an X-ray beam toward an object to be imaged. The X-ray beam may be generated by an X-ray tube or by other techniques. In conventional X-ray imaging systems, the generated X-rays typically have a broad spectrum that may be representative of the technique and/or materials used to generate the X-rays. The generated X-rays typically pass through an imaging volume containing an object or patient. As the X-rays pass through the object or patient, the different materials of which the object or patient are composed attenuate the X-rays to varying degrees. For example, bone, metal, water, air, and soft tissue attenuate the X-rays differently. As attenuated X-rays leave the imaging volume they typically strike a detector where they generate electrical signals that are processed to generate an image of the internal structures of the object or patient.
The X-rays produced in common X-ray tubes are generally of relatively low power, and comprise long pulses or a continuous wave that pose limitations in their use. Moreover, such radiation typically comprises unpolarized, incoherent radiation having a broad energy spectrum. In general, the X-rays generated by conventional techniques may be useful for imaging techniques where the attenuation is measured to produce images, but they are less useful in techniques where energy-dependent information of the materials under inspection are also of interest.
For example, X-ray attenuation through a given object is not constant and is strongly dependent on the X-ray photon energy. This phenomenon manifests itself in an image as a beam-hardening artifact, such as non-uniformity, shading and streaks. Some beam-hardening artifacts can be easily corrected by techniques such as water calibration and iterative bone correction. However, beam hardening from materials other than water and bone, such as metals and contrast agents, are difficult to correct. In addition, the same materials at different locations often show different levels of attenuation. Another limitation of conventional imaging system is lack of material characterization. For example, a highly attenuating material with a low density may result in the same degree of attenuation in the image as a less attenuating material with a high density. Thus, there is little or no information about the material composition of a scanned object based solely on the degree of attenuation. In addition, visibility of certain contrast agents in the human body may be enhanced by imaging the body with properly selected portions of the X-ray spectrum.
Traditional techniques for producing monoenergetic X-ray beams such as fluorescent sources and Bragg angle scattered X-rays for energy selection are employed for various medical applications to overcome the above mentioned limitation. Filtration of a broadband bremsstrahlung radiation can also produce spectra of desired monochromaticity. For example, in mammography, rhodium-coated targets coupled with thin rhodium filtration produces relatively narrow portions of X-ray spectrum centered around the energy of interest. However, in certain cases a significant portion of the X-rays have energies too low to penetrate far into the human body, thereby failing to contribute to an image of the region of interest. In short, a wide X-ray photon energy spectrum from the X-ray source and a lack of energy resolution from the X-ray detectors limit the use of imaging systems for applications such as material characterization, tissue differentiation, scatter rejection and others.
It is therefore desirable to provide an efficient imaging system having monoenergetic X-ray source and energy discriminating detectors to achieve better image contrast and high resolution while minimizing the image noise and radiation doses to the patient.